Method, plant and molds for forming slabs of agglomerate

ABSTRACT

A method for preparation of a mold ( 10 ) intended to form a slab from a mixture of agglomerate comprising the steps of depositing over the mold surfaces a sheet ( 15 ) of PVA-based plastic material so as to form with it a surface for subsequent contact with the mixture introduced into the mold for forming the slab, whereby in at least some areas a layer ( 14 ) of a fluid agent containing PVA in a solution is interposed between the sheet ( 15 ) and the mold surfaces. A plant for carrying out the method and a method for production of a slab are also described.

RELATED APPLICATIONS

This is a continuation of application Ser. No. 16/319,020, filed Jan.18, 2019, which is a 35 U.S.C. 371 national stage filing fromInternational Application No. PCT/IB2017/054265, filed Jul. 14, 2017,which claims priority to Italian Application No. 102016000076298, filedJul. 20, 2016, the teachings of which are incorporated herein byreference.

The present invention relates to a method and a plant for thepreparation of molds and the forming of slabs of agglomerate, forexample agglomerate of stone material granules or powder bonded with ahardening resin. The present invention also relates to a mold preparedusing this method and to a method for production of the slabs.

In the production of slabs of agglomerate, after the material forforming the slabs has been poured into the molds and hardened,preferably after any vacuum vibrocompression, it is necessary toseparate the mold from the slab thus formed.

The molds are preferably made in the form of flexible sheaths ofelastomer material, such as synthetic rubber, and are usually composedof a bottom sheet of rubberized fabric, generally in the form of a traywith edges, and a top closing sheet of rubberized fabric. Such a mold,once filled with an agglomerate mixture, then receives from the outsidea suitable vacuum vibrocompression force in order to shape the slab.

Over time a number of solutions have been developed in an attempt toprevent the hardened sheets from adhering to the mold, in particular tothe bottom rubber sheet and the top rubber sheet of the elastomer mold,and protect the rubber sheets from attack by the resinous binders withany associated solvents.

For example, in IT1311857 it was proposed spraying beforehand onto thesurface of the rubberized sheets a fluid separating/protective agentformed essentially by a solution based on PVA (polyvinyl alcohol),before pouring the mixture into the tray.

After the separating agent has dried, a thin solid and elastic film,which adheres moderately to the surface of the rubberized sheets, isthus formed. The adhesion of the PVA film to the rubberized sheets,although weak, prevents the formation of undulations or wrinkles on thefilm, and therefore on the surface of the slab, during vibrocompressionstep. This film provides protection for the sheets of the mold from theresin and solvents, such as styrene, and also allows separation of thesheets from the hardened slab at the end of the hot catalysis process.

This occurs both for the bottom rubberized sheet onto which the mixtureis poured and for the top rubberized sheet which covers the mixture.

The PVA film instead adheres firmly to the hardened slab and isextracted from the mold together with the slab. During the subsequentprocesses for wet polishing and sizing of the slab the film is thendissolved by the process water and evacuated together with the wetprocessing sludges.

The PVA film may if necessary be mechanically peeled off from the slabinstead of being dissolved in water.

This solution, although effective, has a number of drawbacks associatedwith the fact that the protective/separating agent is fluid so that itmust be applied, normally by means of spraying, and then dried.

In order to obtain a sufficiently compact layer normally a relativelyhigh quantity of fluid, for example about 200 g/m² is applied so as toobtain, after drying, a film of about 30 to 40 g/m².

There is, however, the real risk that the agent may be sprayed notuniformly over the entire mold or that bubbles or voids may be formedand that these, after drying, may produce pores in the film with therisk that solvents may pass through and consequently damage theelastomer mold.

It has also been proposed spraying the fluid agent at a relativelyhigh-temperature (for example about 50° C.) in order to ensure asufficient fluidity, with the risk, however, that the water present init may evaporate excessively during application, with the consequentloss of the fluid characteristics which are required for correctapplication of the layer.

It is however also necessary to clean regularly the nozzles, anoperation which is by no means simple in view of the sticky nature ofthe solution, in order to ensure uniform spraying of the layer.

Moreover, the large amount of solution which is needed for sprayingrequires time and attention during drying in order to eliminate all theconsiderable quantity of water in the solution, while keeping at thesame time the thickness of the sprayed layer sufficiently uniform.

Finally, it should be considered that, after spraying, drying of the PVAsolution must be performed in suitable drying ovens in order to obtainsufficiently rapid drying before being able to pour the mixture into themold. Oven drying is, however, a delicate process which must be carriedout carefully in order to prevent boiling and/or the formation ofbubbles and pitting.

Moreover, the presence of ovens results in an increase in the overalldimensions and costs of the plant and a substantial increase in theamount of electric energy used during the process. The oven acceleratesthe drying process, but complete drying nevertheless requires a certainamount of time.

For these reasons other solutions have been developed, these envisagingprotecting the mold using solid sheets of suitable material which areplaced directly on the bottom and on the walls of the bottom mold beforepouring in the mixture.

The sheet must be chosen so as to have the characteristics necessary foruse in the mold as a means for protecting and facilitating separation ofthe mold from the slab. For example it must be impermeable to organicvapors, in particular styrene, impermeable to the liquid resin,resistant to the catalysis temperatures of the resin, and resistant athigh temperatures to the solvents and chemical vapors which developduring the slab production process.

Sheets of plastic material (polypropylene or PET or PVA) have thereforebeen devised, these being combined where necessary with a sheet of paperhaving an overall gram weight of about 40 to 250 g/m².

After hardening of the layer of mixture, the hardened slab thus obtainedis extracted from the mold together with the sheet which remainsattached thereto. The sheet must then be removed from the slab.

If it is made of PVA, the sheet may be simply dissolved in water at roomtemperature; otherwise it must be removed and eliminated in solid form,i.e. being mechanically removed from the slab.

US2004/169303 describes an example of use of a sheet of water-solubleplastic material. This procedure is also effective but has the drawbackthat the protective sheet, since it simply rests on and does not adhereto the rubberized sheets, may form undulations or wrinkles or foldswhich penetrate into the layer of mixture during vibrocompression of themixture inside the mold and then remain within the hardened slab.

During sizing and polishing of the slabs it is therefore necessary toremove mechanically any trace of paper or plastic film which haspenetrated into the slab. It is clear that, owing to the paper and/orplastic film which has penetrated into the mixture, an extra thicknessof material, which in some cases may also be quite large, must beremoved. Even in the case of PVA which dissolves in water it isnevertheless necessary to remove a layer of material of the slabsufficient to ensure disappearance of the defects which have beencreated in the slab following trapping of the sheet inside the mixture.

This requires, therefore, on the one hand the production of slabs whichare thicker in order to obtain a suitable extra thickness to be removed,and on the other hand, subsequent sizing allowing for a greatermachining depth so that the paper and/or the PVA or polypropylenetrapped in the slab may be removed in order to avoid the consequentdefects.

If this procedure is not adopted, it is required to make do withlow-quality slabs.

The general object of the present invention is to provide a method,plant and a mold for the production of slabs of agglomerates which areable to overcome, among other things, the aforementioned drawbacks ofthe prior art and allow slabs of satisfactory quality to be obtained ina rapid manner.

In view of this object, the idea according to the invention is toprovide a method for the production of slabs from a mixture ofagglomerate, comprising the steps of preparing a mold, intended to forma slab from the mixture of agglomerate, depositing over surfaces of themold at least one sheet of PVA-based plastic material so as to form withit a slab a surface for subsequent contact with the mixture that will beintroduced into the mold, a layer of a fluid agent containing PVA in asolution being interposed between the sheet and at least some areas ofthe mold surfaces.

Still according to the invention, the idea is also to provide a methodfor the production of slabs of agglomerate, comprising the steps ofpreparing a mold using the aforementioned method, introducing into themold a mixture of agglomerate for forming a slab; forming the slab,preferably by means of vacuum vibrocompression; carrying out a processof hardening of the mixture contained in the mold in order toconsolidate the slab; extracting from the mold the hardened slabtogether with the sheet; eliminating the sheet from the slab.Elimination may be advantageously performed by means of simpledissolution.

Still according to the invention, a further idea is to provide a plantfor performing the steps according to the method of the precedingclaims, characterized in that it comprises in sequence a first stationfor application of the layer of fluid agent on at least some areas ofthe surface of the mold and a second station for deposition of the sheetof PVA-based plastic material on the layer, and a conveyor which travelsbetween the first and second stations and is intended to conveysequentially a mold into the first and second stations for applicationof the layer of fluid agent and deposition of the sheet, or,alternatively, displacement of the two stations above the mold which isimmobile.

A further idea is to provide a mold intended to form a slab from amixture of agglomerates, comprising a base portion and a cover having asurface inside the mold which is covered by at least one sheet ofPVA-based plastic material so as to form with it a surface which isintended for subsequent contact with the mixture introduced into themold for forming the slab, between the sheet and at least some areas ofthe said internal surface of the mold there being present a layer of aPVA-containing adhesive agent. The mold may also comprise suction ductswhich are intended to connect a space between surfaces of the mold andthe sheet to external suction means for flattening or stretching thesheet against the mold surfaces by means of a vacuum.

In order to illustrate more clearly the innovative principles of thepresent invention and its advantages compared to the prior art, possibleexamples of embodiment applying these principles will be described belowwith the aid of the accompanying drawings. In the drawings:

FIG. 1 shows a schematic, partial, cross-sectioned view of a moldprepared according to the invention;

FIG. 2 shows a schematic side elevation view of a plant according to theinvention;

FIG. 3 shows a schematic view on a larger scale of a station of theplant shown in FIG. 2;

FIG. 4 shows a partial, schematic, cross-sectional view along the lineIv-Iv of FIG. 3;

FIG. 5 illustrates an operating step of another station of the plantshown in FIG. 2;

FIGS. 6 and 7 illustrate operating steps of the station of the plantshown in FIG. 3;

FIGS. 8 and 9 show operational variants of a plant according to theinvention;

FIG. 10 shows a schematic plan view of a further processing station of aplant according to the invention;

FIG. 11 shows a schematic partial view of a cross-section of the stationaccording to FIG. 10;

FIG. 12 shows a schematic view, on a larger scale, of a detail shown inFIG. 11;

FIG. 13 shows an enlarged schematic view of a detail of the stationshown in FIG. 11 in a different operating position.

With reference to the figures, FIG. 1 shows a partial, schematic,cross-sectional view of a mold, according to the invention, denotedgenerally by 10.

The mold 10 is advantageously a mold made of flexible, preferablypolymeric, material, such as synthetic rubber, including silicone-typerubber, and comprises a bottom part 11 and a top part 12. Cloth orfabric reinforcing layers consisting a substantially non-stretchablematerial (not shown) may also be provided on the outer or inner surfacesor inside the material forming the mold.

The bottom part 11 is preferably realized in the form of a tray, namelywith raised peripheral edges 13 and is suitable for receiving in a diecavity inside the mold a known mixture for forming a slab. The mixturemay be a known mixture of agglomerate, for example an agglomerate ofstone material granules or powder bonded with a hardening resin. Herethe term “die cavity” is understood as referring to the cavity in themold which receives the mixture and which is therefore defined withinthe peripheral edge of the mold by the internal surface of the saidmold.

The top part 12 is preferably substantially flat and forms a cover forclosing the bottom part. Advantageously, the mold may be of the typeused for the known method of forming the slabs by means of vacuumvibrocompression, i.e. once the mixture has been poured in and the moldclosed at the top it is conveyed to a compacting station where itundergoes a vacuum compression action with simultaneous application of avibratory movement. Then the compacted mixture is transferred to astation for hardening the resin, preferably by means of heatapplication.

This type of mold is usually indicated by the technical term “sheath”.

Before distributing the mixture inside the mold, the mold is prepared soas to facilitate the subsequent extraction of the hardened slab andprotect the mold surfaces which otherwise would come into contact withthe mixture.

The preparation comprises the deposition of a sheet 15 of PVA-basedplastic material over the surfaces of the mold 10, be it the bottom part11 or the top part 12, with a layer 14 of a fluid agent containing PVA(polyvinyl alcohol) in a solution interposed between at least some areasof the sheet 15 and the surfaces of the mold as will be explained below.

The sheet 15 forms the surface for subsequent contact with the mixtureintroduced into the mold for forming the slab.

The PVA solution used for the layer 14 may be advantageously formed byPVA in an aqueous solution. It may also envisage, preferably, theaddition of glycerin. This solution may consist, for example (referredto the percentage weight) of about 1% to about 25% of PVA, about 62% toabout 95% of water, and about 1% to about 10% of glycerin.

Additives, such as known anti-foam, leveling and precipitatingadditives, may also be used.

For example, the following compositions were tested (values expressed aspercentage by weight):

Substance Comp.1 Comp.2 Comp.3 Comp.4 Comp.5 Comp.6 Comp.7 H2O 79 66.890.8 86.1 62.1 95.1 71.1 PVA 12.8 25 1 1 25 1 25 Glycerin 5.3 5.3 5.3 1010 1 1 Additives 2.9 2.9 2.9 2.9 2.9 2.9 2.9 TOTAL 100 100 100 100 100100 100

Composition 1 is the composition which is preferred, even thoughcompositions 2 to 7 may be used in accordance with the principles of theinvention. Other compositions may however be used.

It has been found to be particularly advantageous if the layer of fluidagent is applied onto the mold surface by means of mechanical contact ofan applicator element soaked with said fluid agent against the surfaceof the mold. This ensures the application and the consequent formationof a thin uniform thickness of fluid. Application may be performed forexample by means of rolling or sponging.

The average quantity of fluid agent applied may be preferably between 2to 10 g/m², and more preferably between 4 and 6 g/m².

This ensures a sufficient uniformity of coverage without using anexcessive amount of fluid agent, which must have a small thickness(preferably less than 10 μm) in order to ensure a reasonable adhesion ofthe solid PVA film to the surface of the mold and at the same time avoidthe risk of solubilizing and damaging the said film.

However, application by means of spraying, even though difficult toperform in view of the very small thickness, may also be employed.

The PVA-based sheet may be realized in the form of a solid film, forexample with a thickness of between 30 and 50 μm and, preferably, around35 μm.

This sheet may advantageously have a gram weight of between 20 g/m² and60 g/m² and preferably between 30 g/m² and 50 g/m².

After application of the sheet it has been found to be advantageous toperform flattening of the sheet against the mold surface in order toreduce or, preferably, eliminate any voids or air bubbles presentbetween the sheet and the underlying mold surface.

Flattening is particularly advantageous in the case where a subsequentvacuum vibrocompression process is carried out because any bubblespresent underneath the sheet would expand during the vacuum process,causing separation of the film from the sheet.

Advantageously, flattening may be performed by means of a directmechanical action on the sheet or by means of a pneumatic actioninvolving suction of the air between sheet and mold.

In the first case, flattening may be performed advantageously by meansof rolling or brushing, namely using, for example, a soft roller or abrush which are passed over the sheet with suitable pressure. This isadvantageous in the case of shallow mold parts, such as the mold cover.

In the second case, which is preferable for example for deeper moldparts, such as the tray or base of the mold which receives the mixture,the sheet, owing to its extendable nature, may be sucked against themold surfaces by means of suction of the air between the mold and thesheet. In this case, the sheet is preferably fixed beforehand to themold along a peripheral edge, as will become clear below.

Advantageously, the adhesive fluid layer between sheet and at least someareas of the mold ensures an adequate moderate adhesion of the sheet 15to the mold surface. This adhesion is useful for keeping the sheet inposition and for preventing the formation of folds or wrinkles in thesheet during the subsequent processing steps, but at the same time doesnot hinder subsequent voluntary separation of the sheet from the moldafter formation of the slab, as will become clear below.

In fact, the very small quantity of water-based fluid agent forms anadhesive and results in partial limited dissolving of the surface of thesolid PVA film, allowing the necessary adhesion thereof to theunderlying surface of the rubberized mold. The small quantity of waterpresent in the adhesive agent is incorporated within the film structure.

The quantity of the fluid agent is in any case sufficiently small toavoid complete dissolution of the PVA film.

The fluid layer is thus incorporated within the structure of thePVA-based sheet and therefore does not have a negative effect on thelatter, but only ensures the desired adhesion thereof to the mold.

It should also be noted that the use of a fluid adhesive agentconsisting of a PVA solution allows application to be performed in verysmall quantities, without the risk of leaving “patchy zones’, namelyzones which are not suitably covered by the fluid.

The layer of adhesive agent ensures that the sheet adheres sufficientlyto the mold, but also nevertheless ensures subsequent easy removal ofthe slab from the mold.

Advantageously, using a PVA solution and a PVA-based sheet offers theadvantage that they may be subsequently completely removed from the slabsimply by means of washing with water (also during the normal mechanicalslab wet-polishing and finishing operations) or by means of mechanicalpeeling.

There is instead no need for a deep mechanical removal operation sincethe adhesion of the sheet to the mold, as occurs in accordance with theprinciples of the invention, ensures that there is no formation ofwrinkles or folds which may be incorporated in the slab forming mixture.

FIG. 2 shows a schematic view of a plant for preparation of molds,according to the invention, applying the aforementioned method.

For the sake of simplicity in the description below, reference will bemade to the preparation of the bottom part or tray of the mold, but, aswill be clear to the person skilled in the art, the same plant may beused for the similar preparation of the top part or cover of the mold,inserting this top part into the plant in an overturned position withrespect to that shown in FIG. 1. For the sake of convenience belowreference will be made in any case generically to a mold, withoutdistinguishing between the bottom part and top part.

Obviously the same plant may be used for the preparation, in sequence,of the top part and bottom part of the mold, or alternatively two plantsmay be used, i.e. one intended for the preparation of the top parts andthe other one intended for preparation of the bottom parts of the molds.

As can been seen in FIG. 2, the plant (denoted generally by 20)comprises a first station 21 for applying the layer of fluid agent inthe predetermined zones of the mold and a second station 22 fordepositing the sheet of PVA-based plastic material.

A conveyor 23 (preferably of the belt type) is advantageously arrangedbetween the two stations, said conveyor running underneath and betweenthe first and second stations and being intended to transport the moldsequentially into the two stations; alternatively, the two stationscould be displaced above the mold which is kept immobile. In otherwords, the stations may also be designed such that they are mobile andcan be displaced above the mold, instead of displacing the moldunderneath the stations.

A flattening device may be arranged downstream of the sheet applicationzone.

For example, in a first embodiment according to the invention, by way ofan accessory element it may be advantageously envisaged that the secondstation 22 comprises or is followed by a flattening device 24 which,upon passing of the mold, performs mechanical flattening of the sheet 15against the surface of the mold 10.

The first station 21 is intended to distribute uniformity the layer offluid agent on predetermined surfaces of the mold which passesunderneath it, transported by the conveyor 23 and with its surface to betreated directed upwards. Application of the fluid agent allows thesheet to be suitably fixed to the mold for the subsequent processingoperations.

Advantageously, the first station 23 may comprise an applicator element25 which is soaked or coated with the fluid agent so as to transfer thefluid agent by means of contact onto the surface of the mold passingbelow it, in a fluid release zone 26.

In particular, it has been found to be advantageous if the applicatorelement comprises a belt 27 running between a zone 28 for removal of thefluid supplied by a fluid reservoir (for example a fluid tank, notshown), where it is soaked or coated with fluid, and the fluid releasezone 26. The travel movement advantageously occurs in the direction oftravel of the mold underneath the station.

The belt is made with a material suitable for absorbing and/orcollecting a certain quantity of fluid and conveying it to the releasezone 26 where it may be transferred onto the desired areas of the moldsurface.

In a possible preferred embodiment, the belt 27 is wound around tworollers 29 and 30 transverse to the movement of the mold. The firstroller 29 is situated at the bottom so that the belt which is woundaround it is located in the fluid release zone 26 where the fluid comesinto contact with the desired parts of the mold surface to be coated.

The second roller 30 is instead situated in a top position andpreferably in such a way that the belt has at least one section betweenthe two rollers which is inclined towards the direction of advancingmovement of the conveyor 23, with an upwardly directed side which isthat designed to remove the fluid from the fluid reservoir. Means fortransferring onto the belt the fluid to be applied to the mold arepresent along the advantageously inclined section, before the movingbelt 27 reaches the release zone. These means may be of various types,as may be easily imagined by the person skilled in the art

For example, it has been found to be advantageous to use a third roller31, parallel to the first two rollers, placed in contact with theupwardly directed side of the moving belt.

The three rollers and the belt obviously extend transversely withrespect to the conveyor so as to be able to cover the entire width ofthe mold surface to be treated.

As can be seen in FIG. 2, the removal zone 28 for pouring of the fluidis formed between the belt and a third roller, said fluid beingaccumulated between the belt and the third roller and being conveyed insmall quantities by the belt towards the release zone 26.

The third roller 31, in order to facilitate adhesion of the fluid ontothe belt, is preferably arranged so as to be counter-rotating relativeto the belt 27.

The belt basically operates in the manner of a stamping machine whichremoves the fluid and distributes it onto the surface of the mold incontact with the belt in the release zone 26.

Distribution of the fluid therefore essentially takes place by means ofrolling or sponging.

The fluid application device is advantageously provided with pneumaticpistons 32 for being able to adjust the vertical position of theapplicator element so as to allow, for example, the passage of raisedmold parts (for example the rim 13) and so as not to contaminate withthe fluid parts which must not be coated. The device may for example beraised when a mold is not present underneath the station, so as to avoidsoiling the conveyor.

FIGS. 3 and 4 show in greater detail the second station 22 and theflattening device 24, where present.

The second station 22 may comprise a release device which may releasesingle sheets previously prepared to the correct length. However it hasbeen found to be advantageous if the station 22 comprises a feederdevice 16 which releases the sheet 15 in the form of a continuous stripof film (indicated by 15 b) which is unrolled from a reel 33 with itsaxis transverse to the direction of movement of the mold underneath therelease device and then cut to size.

Preferably, unrolling of the plastic film may be performed autonomouslysince the plastic film adheres to the surface of the mold on account ofthe adhesive fluid which has just been applied.

This ensures precise application of the film without wrinkles, due toexcessively fast feeding, or rips or tears, due to slow feeding of thefilm.

In particular, according to a possible structure of the plastic filmfeeder device, the plastic film 15 b unrolled from the reel 33 passesfrom one or more transmission rollers 34 over which the film runs andthen from a final roller 35 which has the function of extending theplastic film over the surface of the mold with which it is in contact.

At least the final roller 35 is movable vertically so as to be able tobe moved towards or away from the underlying conveyor 23 so as to,respectively, press the film with suitable force against the surface tobe covered and/or allow the peripheral rim of the trays to passunderneath, as will be explained more fully below.

In order to perform this vertical movement a pair of pneumatic pistons36, 37 are provided, as can be clearly seen in FIG. 4.

Advantageously, above the reel 33 of plastic film there may be a furtherroller 38, which is preferably soft and may have a brush-like form, saidroller being elastically pushed against the periphery of the reel 33 andhaving the function of smoothing the plastic film, if necessary, inorder to prevent or reduce subsequent trapping of air bubbles betweensheet and deposition surface inside the mold and prevent wrinkling ofthe sheet before it is applied onto the mold. The roller 38 may alsoprevent uncontrolled unrolling of the reel, maintaining a correcttension of the plastic film between the reel and the following rollers34 and 35.

In order to perform cutting to size of the strip of film for forming thesheet on the mold, advantageously a cutting device 39 is provided, saiddevice being arranged transverse to the strip of unrolled film andadvantageously situated downstream of the roller 35.

In particular, it has been found to be advantageous to provide thecutting device 39 with a wire 40 heated to the temperature needed tomelt the material of the film and arranged transverse to the directionof feeding of the mold. Motor-driven means 41 (for example one or morepneumatic pistons) move upon command the wire 40 between annon-operative raised position, where it does not interfere with theplastic film, and an operative, lowered, cutting position where the wireis arranged against the plastic film. As can be seen in FIG. 4, the wireis advantageously supported and tensioned between its two ends 42, 43which are situated on the sides of the conveyor 23 and on which themotor-driven means 41 act via connections and linkages 44 and 45. Themovement of the wire between its two—non-operative andoperative—positions may be vertical so as to be lowered onto the filmonce the film has been deposited onto the mold.

Advantageously the wire is metallic and heated by means of the Jouleeffect via an electric current which flows in it thanks to an electricpower supply (not shown) connected to its two ends 42 and 43.

As can be clearly seen again in FIG. 3, the optional flattening device24 (used in a first possible embodiment of the invention to apply amechanical flattening action) comprises preferably a pressure roller 46with the axis of rotation which is transverse to the direction ofmovement of the conveyor and which is pushed towards the conveyor so asto act on the sheet deposited on the mold surface and press it againstthe mold surface when the mold passes underneath it. This pushing actionmay be obtained, for example, by means of suitable elastic supporting ofthe roller 46, the weight itself of the roller, a yielding design of theperipheral surface of the roller, or a combination of these threecharacteristics.

The pressure roller 46 may, for example, be made with a peripheralsurface which is soft and compressible or in the form of a brush.

During use of the plant, the mold part (tray or cover) to be treated ismoved on the conveyor 23 with the side to be treated directed upwards,as shown schematically in FIG. 2. As already mentioned, the figuresrelating to the plant show for the sake of simplicity the bottom moldpart or tray 11. A similar treatment may be applied to the top part orcover 12 of the mold which must close the mixture inside the mold.

Initially both the station 21 for applying the layer of fluid agent andsecond station 22 for depositing the sheet of plastic material are intheir rest position, namely respectively have the fluid applicatorelement 25 and the plastic film feed device raised.

When the front end of the mold (front end and rear end are understood asbeing in relation to the movement of the mold along the plant) reachesthe first station 21, the fluid applicator element 25 is lowered so thatit comes into contact with the surface of the mold to be treated withthe fluid.

FIG. 5 shows a first possible application procedure according to theinvention. According to this first procedure, the applicator element 25is lowered so as to come into contact substantially with the wholesurface of the mold which must then receive the sheet (where necessaryavoiding only the rim of the tray 11). In particular, the fluid isapplied onto the bottom of the mold.

The advancing movement of the mold allows the deposition of the desiredfluid layer over the whole surface of the mold to be treated, as can beclearly seen in FIG. 5. When the rear end of the mold passes by theapplicator element 25 is raised and application of the fluid terminates.

When the front end of the mold reaches the second station 22 the devicefor feeding the plastic film 16 is operated so as to start dispensing ofthe plastic sheet. In particular, in case of the plant shown in FIG. 6,the roller 35 is lowered so that the front portion of the plastic filmis pushed towards the surface of the mold covered with fluid.

In this condition, the advancing movement of the mold allows the film tobe conveyed and deposited on the mold surface, as can be clearly seen inFIG. 6. Once the rear end of the mold has passed by the roller 35 israised and application of the film terminates.

As can be seen again in FIG. 6, in the case where the flattening device24 is present, it is operated so as to cause the plastic film to adhereto the surface of the mold, thus preventing the trapping of air bubbles.

As can be seen in FIG. 7, once application of the plastic film has beencompleted, the roller 35 is raised and remains in this position. Thecutting device 39 is instead operated so as to cut and separate the partof film which has been dispensed onto the mold, creating a new frontportion for the next dispensing operation which takes place upon arrivalof a new mold.

The flattening device instead continues its action and completes theoperation.

As already mentioned above, the application of the fluid over the wholesurface of the mold which must be covered by the sheet is preferable inthe case of a shallow mold. For example, the procedure described abovemay be advantageously used for covering the cover of the mold.

In the case of deeper molds, purely mechanical application of the sheetmight not always be satisfactory.

In this case it has been found to be advantageous to perform “pneumatic”flattening of the sheet, using a flattening device which is arrangeddownstream of the sheet application zone and which acts by means of avacuum system instead of a system with the aforementioned mechanicaldevice 24.

In the case of a vacuum flattening action it has also been found to beadvantageous if the fluid is applied only on some zones of the moldsurfaces which must be covered by the sheet. In particular, it has beenfound to be advantageous to apply the fluid only in peripheral zones ofthe mold, so as to define a central area from which the air may besucked.

This variant of the method according to the invention is shown by way ofexample in FIG. 8.

According to this variant, when the front end of the mold reaches thefirst station 21, the fluid applicator element 25 is lowered so as tocome into contact with only the peripheral edge or the rim of the mold.In this way, in the first working station, the PVA-based adhesive isdistributed using the method already described above, but only on thetop peripheral edge of the mold (for example the tray).

As can be clearly seen in FIG. 9, the mold thus treated then passes tothe sheet deposition station 22, where the sheet is stretched over themold so as to adhere to the peripheral rim of the mold treated with theadhesive fluid. In this way, the sheet is arranged so as to remainhorizontal and spaced from the bottom of the mold and, owing to thePVA-based adhesive and its lightness, it remains perfectly in position(like the skin of a drum) over the mold die cavity.

In the station 22 the optional flattening device 24 is not used (and maytherefore also not be present, as can be seen from FIG. 9). If, instead,air suction is subsequently used, advantageously it is performed viasuction ducts arranged in the mold so as to emerge inside the moldinterior defined by the peripheral edge of the mold. Suction means willtherefore be provided instead of the flattening device 24.

In particular, the mold 11 with the sheet applied may pass to afollowing station 50, called mixture distribution and spreading station,shown schematically in a top view in FIG. 10 and in a cross-sectionalview in FIG. 11.

In this station the mixture is normally poured into the mold, thestation usually comprising transportation means 51 (for example a knownbelt conveyor) which moves the mold 11 opposite a known distributionunit 52 which is able to distribute the mixture inside the mold in thecorrect amount. In the case where the solid film is positioned insidethe mold according to the first embodiment of the present invention, thestation 50 will be substantially of the known type, as may be easilyimagined by the person skilled in the art. The mixture is generallypoured into the mold by means of a longitudinal relative movement of themold and distribution unit 52, with the distribution unit extendingtransversely over the whole width of the mold. The distribution unit andthe relative movement with respect to the mold are known per se and willtherefore not be illustrated or described here further, being able to beeasily imagined by the person skilled in the art.

Preferably, it will be the unit 52 which moves along the mold, which iskept stationary inside the station 50. However a movement of the moldunderneath the unit 52 which remains stationary, or a combined movementof mold and unit, may also be used if desired.

The distribution will be advantageously performed by means of a known“loosening” action where the mixture is spread out so as to level thefree surface of the mixture inside the mold.

According to the invention, before pouring of the mixture, flattening ofthe sheet which has been arranged on the mold during the alreadypreviously described step is performed.

If the film has been fixed on the top peripheral edge of the mold andnot yet flattened, as shown in FIG. 9, the vacuum flattening orstretching device 53 may be directly present in the station 50 so as toperform the flattening/stretching step by means of suction of the airtrapped in between the sheet and mold before and in some cases alsoduring distribution of the mixture.

For this purpose, the spreading station 50 may advantageously comprisesuction means which form the vacuum flattening device 53 and which aresuitably connected to the mold 11 reaching the station, so as to removethe air trapped between the sheet 15 and the mold surface.

In particular, the suction means may be arranged in the station 50 alongperipheral edges of the mold so as to link up (connect) with suitablesuction ducts present in the said mold (ducts which have for example adiameter of about 6-10 mm) and which connect the internal cavity of themold underneath the sheet 15 to the exterior of the mold.

Advantageously, the edges of the mold where the suction means arearranged may be the two opposite longitudinal edges so that the mold maybe arranged between the suction means 53 by means of the sametransportation movement which brings the mold inside the station 50.

The passages inside the mold and the associated suction means may beadvantageously arranged at intervals along the edges of the mold, as isclearly visible in FIG. 10, so as to suck uniformly the air trappedunderneath the sheet applied to the mold. For example, the suction meansmay be formed by four connectors on one side and four connectors on theopposite side of the mold.

Again advantageously, the bottom of the mold may have preferably asurface which is not smooth so as to allow better evacuation of the airbetween the bottom of the mold and the solid PVA film. In order toachieve this, the rubber face of the mold on which the mixture isdeposited with the PVA sheet in between instead of being smooth may havea suitable surface roughness, this being achieved for example by usingpreferably a surface cloth or rubber with a knurled imprint or the like.

Owing to the roughness of the internal surface of the mold,micro-channels for passage of the air are created, these allowing theair to be sucked not only close to the rim of the mold, but also intothe centre of the mold (or tray) such that the solid PVA film restsperfectly over the whole of the bottom of the tray without the risk ofcausing any puckering.

Moreover, if the internal surface of the mold is made of cloth, the PVAfilm adheres better to the bottom of the tray since the cloth has bettergripping properties than rubber.

FIG. 12 shows an enlarged view of an advantageous embodiment of thesuction means (in the zone indicated by a broken line circle in FIG. 11)which are designed so as to allow automatic controlled engagementthereof with the aforementioned suction ducts present in the mold.

As can be seen in FIG. 12, the suction ducts (indicated generally by 54)preferably extend so as to emerge laterally inside the internal cavityof the mold, close to the bottom, and thus allow the correct suction ofthe air even when the PVA sheet starts to move towards the bottom of themold.

Again in FIG. 12 it can be seen how the suction means 53 may comprisefor each duct 54 a sealed connecting end 55 (for example in the form ofa suction cup) which, by means of a suitable actuator 56 (for example apneumatic actuator), is pushed against the end of the duct 54 whichemerges outside the mold. Advantageously, the actuator 56 moves thesealed connecting end 55 horizontally between a retracted position,shown in solid lines in FIG. 12, and an advanced operative position,shown in broken lines in FIG. 12, so as to form a seal against the outerend of the duct 54.

The connecting end 55 is in turn connected to a controlled vacuum sourceor vacuum plant (not shown) so as to suck the air into the internalcavity of the mold through the passage 54. Advantageously, the end ofthe duct 54 which opens out inside the mold interior is provided with asuitable filter 57 which allows the air to pass through, but preventsfor example the sheet 50 from being sucked into the duct 54.

During operation of the plant, when a mold (prepared in accordance withthat described with reference to FIGS. 8 and 9) reaches the spreadingstation 50, the suction means 53 are operated so as to be connected tothe ducts 54 present in the mold and suck the air between the bottom ofthe mold (for example in the form of a tray) and the solid film 15applied to the edge of the mold.

Consequently, a solid film owing to its extendable nature, stretches andrests against the inner surface of the rim and the bottom of the mold soas to assume its final form, as shown in broken lines in FIG. 12.

The suction time needed for the solid film to be arranged perfectlyagainst the inner surface of the mold may be reduced to only a fewseconds. However, for greater reliability, suction may be maintained fora longer period of time before pouring the mixture. Moreover, suctionmay be maintained also during the entire mixture distribution andspreading operation so as to ensure that the sheet 15 remains stableagainst the bottom of the mold.

Once the mixture has been poured air suction may be interrupted sincethe film previously shaped by suction retains its form adhering to thesurface of the mold since it is acted on by the weight of the mixturedistributed over it.

The spreading station 50 may also preferably comprise a peripheral rim58 for containing the mixture, which is lowered via drive means (notshown) by a frame 59 and placed on the bottom of the tray in thevicinity of its peripheral edge before the mixture is poured. The rim isshown for example in the raised position in FIG. 11 and in the loweredposition inside the mold in the enlarged view of FIG. 13. The rim mayalso form part of the distribution unit 52 so as to move, at leastvertically, together with it.

The function of the peripheral rim is to help contain the mixture whenit is distributed inside the tray and prevent part of it fromaccidentally spilling outside of the tray.

Once the containing rim has been lowered, with suction of the air stillactive, the mixture is distributed inside the tray. Once distribution ofthe mixture has been completed, the peripheral containing rim may beraised again and the mold, disconnected from the suction means, may beconveyed away for the subsequent slab production steps.

In any case, once the mold has been filled with the mixture, it may beclosed at the top by the appropriate cover, advantageously treatedbeforehand with PVA film applied for example as already described above.For example, a closing station (substantially known per se and thereforenot shown) may pick up the cover, turn it over and deposit it on top ofthe mixture contained inside the tray.

The tray thus closed and containing the mixture may then be transferredto a forming station (also substantially known per se and therefore notshown) where, preferably, a known step of vacuum vibrocompression of themixture is performed.

The internal filters 57 present inside the ducts 54 the mold also havethe function of preventing the mixture, during vibrocompression, frombeing pushed inside the passages, tearing the PVA film and/or creatingprotrusions which would then prevent extraction of the slab from thetray after hardening.

At the end of the production line the covers are finally removed so asto be able to remove the hardened slab. A return line for the covers andtrays may also be provided so that they may be cleaned and then usedagain for application of the solid PVA film, as previously described,and the mold preparation and slab production cycle repeated.

The slabs, after catalysis and hardening, may instead be conveyed awayfor removal of the PVA sheets. The sheets may be peeled off mechanically(manually or using automatic devices) from the said slab or the slabsmay be transferred to a wet-polishing and sizing line with the sheetstill applied, where processing water causes dissolving of the sheetswhich are then removed together with the processing sludges.

At this point it is clear how the predefined objects have been achieved.Owing to the principles of the invention, the surfaces of the mold assuitably protected, the slab produced may be easily separated from themold, and sufficient adhesion of the protective sheet to the mold duringpreparation and use of the mold is ensured.

Obviously the description provided above of embodiments applying theinnovative principles of the present invention is provided by way ofexample of these innovative principles and must therefore not beregarded as limiting the scope of the rights claimed herein.

For example, depending on the length of the molds and the distancebetween stations, the steps of applying the fluid layer and depositingthe solid layer may be completely separated temporally or partiallyoverlapped, in the sense that, while the fluid application station isoperating on the rear part of the mold, the sheet deposition station isalready operating on the front part of the mold.

In any case, once deposition and any flattening has terminated, the moldmay leave the preparation plant and continue towards the following slabproduction steps, if necessary also after a temporary storage period.

As mentioned above, during the following steps, preferably the mixtureof agglomerate forming a slab will be distributed in the mold and thelatter, if necessary, closed at the top with the cover; the slab will beformed; the slab allowed to undergo the hardening process; the slabextracted from the mold together with the sheet or sheets; and the sheeteliminated from the slab.

The forming process may in turn comprise a known procedure forcompaction by means of vibrocompression, optionally under a vacuum, andsubsequent hot hardening of the mixture.

Elimination of the sheet may be performed mechanically and/or by meansof dissolving of the sheet in water (for example during the mechanicalprocess for finishing the slab surfaces which generally involveswet-polishing and sizing).

All these subsequent processing stages are not shown or described herein greater detail, since they are known per se and may be easilyimagined by the person skilled in the art in the light of thedescription provided here.

Owing to the principles of the invention rubber trays may be used, withall the associated advantages, and it is ensured that parts or traces ofthe protective sheet do not remain trapped in the slab, even if themixture undergoes vibrocompression.

During sizing and smoothing of the hardened slab, the extra thickness ofmaterial to be removed will therefore be smaller in nature.

As a result the thickness of the layer of mixture to be distributedinside the tray may be reduced and, therefore, the quantity of mixturestrictly needed for each slab may be used. There is also a reduction inthe energy costs and costs of tools needed for processing of the slab,as well as a reduction in the amount of processing sludges.

Although the application of the fluid by means of contact, in particularusing a device with an applicator element has described above, has beenfound to be preferable in order to obtain more uniform release of thelayer of fluid and greater control over its small thickness, the firststation 21 for applying the fluid agent layer may also alternativelycomprise suitable nozzles located in the fluid release zone so as tospray the fluid directly towards the surface of the mold which ispassing through the zone. Moreover, other known contact applicationelements and devices may be used.

The application of the fluid between mold and sheet and the applicationof the sheet may also be performed at much shorter time intervals and ina much smaller space compared to that shown and described. For example,the fluid may be applied directly in the sheet deposition station,immediately before the sheet touches the mold surface to which it mustadhere, so as to be arranged between the sheet and the mold surface andnot necessarily spread beforehand over the mold surface.

The form, dimensions and proportions of the various parts of the moldand the plant may vary with respect to those shown here, depending onthe specific requirements. Moreover, the plant may comprise knownaccessory elements for management thereof, synchronization of theoperations, detection of the passage of the mold parts for activation ofthe various mechanisms, etc.

In order to protect very wide molds, two or more rolls of solid filmarranged in parallel may also be used, so that the film of each rollcovers only a part of the surface of the mold. This could be useful forexample in the case where slabs of considerable width must be produced.

1. Base portion of a mold usable for forming a slab from a mixture ofagglomerate, comprising: an internal surface adapted to be covered by atleast one sheet of plastic material (15) to form a surface to contactwith the mixture of agglomerate when introduced into the mold (10) forforming the slab; and at least one suction duct (54) connecting a spacedefined between the internal surface of the base portion (11) and the atleast one sheet of plastic material (15) to an external suction means(53) usable to suck air from between the at least one sheet of plasticmaterial (15) and the mold to flatten or stretch the at least one sheetof plastic material (15) against the internal surface of the baseportion (11) by means of vacuum.
 2. The base portion according to claim1, wherein the at least one suction duct comprises a plurality ofsuction ducts (54).
 3. The base portion according to claim 2, furthercomprising a filter (57) arranged at one end of each of the plurality ofducts, each filter leading into the base portion.
 4. The base portionaccording to claim 1, wherein the internal surface of the base portionis at least partially made with one of a cloth or of rubber, with aknurled surface.
 5. Mold intended to form a slab from a mixture ofagglomerate comprising: a base portion (11) comprising an internalsurface adapted to be covered by at least one sheet of plastic material(15) to form a surface to contact with the mixture of agglomerate whenintroduced into the mold (10) for forming the slab, the base portionoperably connected to an external suction means (53) usable to suck airfrom between the at least one sheet of plastic material (15) and themold to flatten or stretch the at least one sheet of plastic material(15) against the internal surface by means of vacuum; and a cover (12)configured to be superimposed to the base portion (11).
 6. The mold ofclaim 5, wherein the base portion further comprises at least one suctionduct (54) connecting a space defined between the internal surface of thebase portion (11) and the at least one sheet of plastic material (15) tothe external suction means (53).
 7. The mold according to claim 6,wherein the at least one suction duct comprises a plurality of suctionducts (54).
 8. The mold according to claim 7, further comprising afilter (57) arranged at one end of each of the plurality of ducts, eachfilter leading into the base portion.
 9. The mold according to claim 6,wherein the internal surface of the base portion is at least partiallymade with one of a cloth or of rubber, with a knurled surface.
 10. Asystem comprising: at least one sheet (15) of plastic material (15); amold (10) comprising a base portion (11) having an internal surface; anda station (22) configured for depositing the at least one sheet ofplastic material (15) on the internal surface; and an external suctionmeans (53) usable to suck air from between the at least one sheet ofplastic material (15) and the mold to flatten or stretch the at leastone sheet of plastic material (15) against the internal surface by meansof vacuum.
 11. The system according to claim 10, wherein the at leastone sheet of plastic material (15) is PVA based.
 12. The systemaccording to claim 10, further comprising a fluid adhesive agent, and afurther station (21) configured for applying a layer of the fluidadhesive agent to at least some areas of the internal surface of themold.
 13. The system according to claim 12, wherein the depositingstation (22) follows the applying station (21), the depositing stationconfigured for depositing the sheet of plastic material on the layer ofthe fluid adhesive agent.
 14. The system according to claim 10, whereinthe base portion of the mold comprises at least one suction duct (54)connecting a space defined between the internal surface of the baseportion (11) and the at least one sheet of plastic material (15) to theexternal suction means (53).
 15. The system according to claim 14,wherein the at least one suction duct comprises a plurality of suctionducts (54).
 16. The system according to claim 15, further comprising afilter (57) arranged at one end of each of the plurality of ducts, eachfilter leading into the base portion.
 17. The system according to claim10, wherein the depositing station (22) comprises a device (33, 34, 35)for releasing the at least one sheet of plastic material (15) in theform of a strip of film (15 b) which is unrolled from a reel (33) and adevice (39) for transversely cutting the strip of film to form the atleast one sheet of plastic material (15).
 18. The system according toclaim 10, wherein the depositing station (22) comprises or is followedby a flattening device (24, 53) for smoothing the sheet against theinternal surface of the mold.
 19. The system according to claim 18,characterized in that the flattening device comprises the externalsuction means (53) for sucking the air between sheet and mold.
 20. Thesystem according to claim 19, characterized in that the suction means(53) are arranged in another station (50) and are movable on command soas to connect to/disconnect from suction ducts (54) present in the mold(10).